1. Field of the Invention
The present invention relates to a line noise attenuator which purifies electromagnetic environments by attenuating noise currents superimposed on lines for supplying electric powers or signals to an electronic device from a power source or a signal source.
2. Description of the Related Art
Noise currents superimposed on conductor lines such as earth lines and power source lines connected to electronic devices mainly are common mode currents which are generated because those conductor lines operate as antennas. Regarding parallel conductor lines such as power source lines constituted with those conductor lines, wiring modes are not uniform. Those lines are drawn around by changing the directions, and a part of the common mode currents generating discontinuous points in the parallel conductor lines are converted into normal mode currents.
It is necessary to have a noise attenuator for eliminating and lightening obstructions caused due to both noise currents of common mode and normal mode superimposed on various kinds of conductor lines. Patent Document 1 discloses this type of noise attenuator.
The noise filter disclosed in Patent Document 1 is structured to prevent a resonance phenomenon with a capacitor for eliminating normal mode noise and a choke coil for suppressing harmonic currents. The noise attenuator extends passing time of input currents in a low frequency region by the choke coil to reduce a peak value of the input currents so as to suppress a harmonic component. Further, the normal noise is absorbed by the capacitor within a power source device, i.e., the noise in the normal mode is reflected upon an AC power side via the capacitor and a feedback line. Furthermore, a series resonance phenomenon of the choke coil and the capacitor is suppressed by connecting a resistance in parallel to the choke coil.
Further, Patent Document 2 discloses a double-terminal safety earth line noise filter which is attached to earth lines of electronic devices whose ground capacitances vary for each device.    Patent Document 1: Japanese Unexamined Patent Publication H8-265085    Patent Document 2: Japanese Patent No. 3957206
The noise filter disclosed in Patent Document 1 overcomes such an issue that the seventeenth to twenty-third harmonic currents exceed a limit value for satisfying the standard because of the series resonance phenomenon of the capacitor and the choke coil. Through adding a resistance to a combination of the normal mode noise eliminating capacitor and the harmonic current suppressing choke coil, the seventeenth to twenty-third harmonic currents are improved to be equal to or less than the limit value.
However, in the case of Patent Document 1, the normal mode noise is suppressed by the capacitor. Thus, voltages at both ends of the capacitor become same voltages in the case of common mode. Therefore, it is not possible with this to make a short-circuit path by the capacitor and to reflect the high-frequency noise current to the AC power source side.
Further, even though the resistance is connected in parallel to the choke coil, the resistance is for suppressing the series resonance phenomenon generated with the capacitor. Thus, the resistance value thereof is limited to a value which can suppress the series resonance phenomenon. It is true that the power is consumed by the resistance when electric currents flow into the resistance. However, the power consumption amount is set for suppressing the series resonance phenomenon. Thus, the resistance value is suppressed low. Therefore, the noise current cannot be consumed up to the value for suppressing the noise obstruction, even if the normal mode noise flows into the resistance.
Further, in Patent Document 1, inductance of the choke coil and the resistance value of the resistance are obtained based on actually measured data. Regarding the power source line, the wiring modes are not uniform as described above, and it is drawn around by changing the directions, thereby generating discontinuous points. Thus, the technique disclosed in Patent Document 1 is designed assuming that a part of the common mode noise current may be converted to normal mode noise current. When most of the noise currents are of common mode, it is not possible with the method of Patent Document 1 to reflect the noise to the AC power source side by the capacitor since the voltages at both ends of the capacitor are the same voltages as described above. Therefore, it is difficult to deal with the noise obstructions in the common mode.
Further, Patent Document 2 utilizes magnetic saturation of an inductor, and it is structured to magnetic-saturate the inductor with low-frequency currents of a large current value which flows into an apparatus and causes malfunctions to the apparatus so as to protect the apparatus by discharging the low-frequency currents to the earth, and to heat-consume the high-frequency noise small currents superimposed on the earth line with the resistance provided in parallel to the inductor. When the technique of Patent Document 2 is applied as it is to a power system, the inductor is magnetic-saturated since the low-frequency power source current flown to the conductor of the power system is of a large current value. Thus, the high-frequency noise current superimposed on the conductor flows into an apparatus without being blocked by the inductor. This results in causing malfunctions in the apparatus by the high-frequency noise current.
Therefore, in order to build the technique of Patent Document 2 as a noise filter for the power system, it is necessary to establish a design theory suited for the noise filter for the power system from a different point of view. Looking into inductors that are actually commercialized, no inductor having an inductance value required for blocking the high-frequency noise current without causing magnetic saturation has been developed. Therefore, it is necessary to develop an inductor that can overcome such issue.